The dual role of tetraspanin CD63 in HIV-1 replication

Enhanced capture of preeclampsia-derived extracellular vesicles from maternal plasma by monocytes and T lymphocytes

Released from trophoblast and other fetal cells, placental extracellular vesicles (EVs) reach the maternal peripheral blood and modulate immune responses. Increased EVs in plasma of preeclampsia (PE) patients indicate their involvement in the etiology of this condition. This study addresses the uptake of plasma EVs by peripheral blood mononuclear cells (PBMCs) and explores the underlying internalization mechanisms. Plasma EVs were isolated from women with normotensive pregnancy (EVNP) and those with PE (EVPE), and characterized by cryo-transmission electron microscopy, nanoparticle tracking analysis, Western blotting, flow cytometry, and micro bicinchoninic acid assay (micro-BCA). To investigate whether the origin of PBMCs affects uptake, samples from males, pregnant women, and non-pregnant women were included. Primary PBMCs and macrophages derived from the human leukemia monocytic cell line THP-1 were incubated with PKH-stained EVs, and uptake was assessed by flow cytometry and confocal microscopy. Key molecules involved in monocyte differentiation and macrophage function were evaluated in EV-treated cells using LEGENDplex™ assay and real-time polymerase chain reaction (RT-PCR). Independent of the PBMC source, EVs were mostly captured by monocytes and in a lower proportion by T lymphocytes. Capture of EVPE was higher than of EVNP in primary T lymphocytes, monocytes, and THP-1-derived macrophages. After inhibition by Wortmannin and Cytochalasin D, EV internalization by THP-1-derived macrophages was significantly inhibited but not completely abolished. No defined polarization profile of treated THP-1-derived macrophages could be identified. These findings provide evidence of EV modifications in PE, which enhance their uptake by monocytes and other immune cells, mainly through phagocytosis and endocytosis.

Targeting Tetraspanins at Cell Interfaces: Functional Modulation and Exosome-Based Drug Delivery for Precise Disease Treatment

Tetraspanins are key players in various physiological and pathological processes, including malignancy, immune response, fertilization, and infectious disease. Affinity ligands targeting the interactions between tetraspanins and partner proteins are promising for modulating downstream signaling pathways, thus emerging as attractive candidates for interfering related biological functions. Due to the involvement in vesicle biogenesis and cargo trafficking, tetraspanins are also regarded as exosome markers, and become molecular targets for drug loading and delivery. Given the rapid development in these areas, this minireview focuses on recent advances in design and engineering of affinity binders toward tetraspanins including CD63, CD81, and CD9. Their mechanism of actions in modulating protein interactions at cell interfaces and treatment of malignant diseases are discussed. Strategies for constructing exosome-based drug delivery platforms are also reviewed, with emphasis on the important roles of tetraspanins and the affinity ligands. Finally, challenges and future development of tetraspanin-targeting therapy and exosomal drug delivery platforms are also discussed.

Extracellular vesicle isolation methods identify distinct HIV-1 particles released from chronically infected T-cells

The current study analyzed the intersecting biophysical, biochemical, and functional properties of extracellular particles (EPs) with the human immunodeficiency virus type-1 (HIV-1) beyond the currently accepted size range for HIV-1. We isolated five fractions (Frac-A through Frac-E) from HIV-infected cells by sequential differential ultracentrifugation (DUC). All fractions showed a heterogeneous size distribution with median particle sizes greater than 100 nm for Frac-A through Frac-D but not for Frac-E, which contained small EPs with an average size well below 50 nm. Synchronized and released cultures contained large infectious EPs in Frac-A, with markers of amphisomes and viral components. Additionally, Frac-E uniquely contained EPs positive for CD63, HSP70, and HIV-1 proteins. Despite its small average size, Frac-E contained membrane-protected viral integrase, detectable only after SDS treatment, indicating that it is enclosed in vesicles. Single particle analysis with dSTORM further supported these findings as CD63, HIV-1 integrase, and the viral surface envelope (Env) glycoprotein (gp) colocalized on the same Frac-E particles. Surprisingly, Frac-E EPs were infectious, and infectivity was significantly reduced by immunodepleting Frac-E with anti-CD63, indicating the presence of this protein on the surface of infectious small EPs in Frac-E. To our knowledge, this is the first time that extracellular vesicle (EV) isolation methods have identified infectious small HIV-1 particles (smHIV-1) that are under 50 nm. Collectively, our data indicate that the crossroads between EPs and HIV-1 potentially extend beyond the currently accepted biophysical properties of HIV-1, which may have further implications for viral pathogenesis.

Phospho-eIF4E stimulation regulates coronavirus entry by selective expression of cell membrane-residential factors

The eukaryotic translation initiation factor eIF4E can regulate cellular translation via phosphorylation on serine 209. In a recent study, by two rounds of TMT relative quantitative proteomics, we found that phosphorylated eIF4E (p-eIF4E) favors the translation of selected mRNAs, and the encoded proteins are mainly involved in ECM-receptor, focal adhesion, and PI3K-Akt signaling. The current paper is focused on the relationship between p-eIF4E and the downstream host cell proteins, and their presumed effect on efficient entry of PEDV. We found that the depletion of membrane-residential factor TSPAN3, CD63, and ITGB2 significantly inhibited viral invasion of PEDV, and reduced the entry of pseudotyped particles PEDV-pp, SARS-CoV-pp, and SARS-CoV-2-pp. The specific antibodies of TSPAN3, CD63, and ITGB2 blocked the adsorption of PEDV into host cells. Moreover, we detected that eIF4E phosphorylation was increased at 1 h after PEDV infection, in accordance with the expression of TSPAN3, CD63, and ITGB2. Similar trends appeared in the intestines of piglets in the early stage of PEDV challenge. Compared with Vero cells, S209A-Vero cells in which eIF4E cannot be phosphorylated showed a decrease of invading PEDV virions. MNK kinase inhibitor blocked PEDV invasion, as well as reduced the accumulation of TSPAN3, CD63, and ITGB2. Further study showed that the ERK-MNK pathway was responsible for the regulation of PEDV-induced early phosphorylation of eIF4E. This paper demonstrates for the first time the connections among p-eIF4E stimulation and membrane-residential host factors. Our findings also enrich the understanding of the biological function of phosphorylated eIF4E during the viral life cycle.IMPORTANCEThe eukaryotic translation initiation factor eIF4E can regulate cellular translation via phosphorylation. In our previous study, several host factors susceptible to a high level of p-eIF4E were found to be conducive to viral infection by coronavirus PEDV. The current paper is focused on cell membrane-residential factors, which are involved in signal pathways that are sensitive to phosphorylated eIF4E. We found that the ERK-MNK pathway was activated, which resulted in the stimulation of phosphorylation of eIF4E in early PEDV infection. Phospho-eIF4E promoted the viral invasion of PEDV by upregulating the expression of host factors TSPAN3, CD63, and ITGB2 at the translation level rather than at the transcription level. Moreover, TSPAN3, CD63, or ITGB2 facilitates the efficient entry of coronavirus SARS-CoV, SARS-CoV-2, and HCoV-OC43. Our findings broaden our insights into the dynamic phosphorylation of eIF4E during the viral life cycle, and provide further evidence that phosphorylated eIF4E regulates selective translation of host mRNA.

Antiviral HIV-1 SERINC restriction factors disrupt virus membrane asymmetry

The host proteins SERINC3 and SERINC5 are HIV-1 restriction factors that reduce infectivity when incorporated into the viral envelope. The HIV-1 accessory protein Nef abrogates incorporation of SERINCs via binding to intracellular loop 4 (ICL4). Here, we determine cryoEM maps of full-length human SERINC3 and an ICL4 deletion construct, which reveal that hSERINC3 is comprised of two α-helical bundles connected by a ~ 40-residue, highly tilted, "crossmember" helix. The design resembles non-ATP-dependent lipid transporters. Consistently, purified hSERINCs reconstituted into proteoliposomes induce flipping of phosphatidylserine (PS), phosphatidylethanolamine and phosphatidylcholine. Furthermore, SERINC3, SERINC5 and the scramblase TMEM16F expose PS on the surface of HIV-1 and reduce infectivity, with similar results in MLV. SERINC effects in HIV-1 and MLV are counteracted by Nef and GlycoGag, respectively. Our results demonstrate that SERINCs are membrane transporters that flip lipids, resulting in a loss of membrane asymmetry that is strongly correlated with changes in Env conformation and loss of infectivity.

Lack of involvement of CD63 and CD9 tetraspanins in the extracellular vesicle content delivery process

Extracellular vesicles (EVs) are thought to mediate intercellular communication by transferring cargoes from donor to acceptor cells. The EV content-delivery process within acceptor cells is still poorly characterized and debated. CD63 and CD9, members of the tetraspanin family, are highly enriched within EV membranes and are respectively enriched within multivesicular bodies/endosomes and at the plasma membrane of the cells. CD63 and CD9 have been suspected to regulate the EV uptake and delivery process. Here we used two independent assays and different cell models (HeLa, MDA-MB-231 and HEK293T cells) to assess the putative role of CD63 and CD9 in the EV delivery process that includes uptake and cargo delivery. Our results suggest that neither CD63, nor CD9 are required for this function.

LAMP3/CD63 Expression in Early and Late Endosomes in Human Vaginal Epithelial Cells Is Associated with Enhancement of HSV-2 Infection

Herpes simplex virus 2 (HSV-2) is a lifelong sexually transmitted virus that disproportionately infects women through heterosexual transmission in the vaginal tract. The vaginal epithelium is known to be highly susceptible to HSV-2 infection; however, the cellular mechanism of HSV-2 uptake and replication in vaginal epithelium has not been extensively studied. Previously, we observed that lysosomal-associated membrane protein-3 (LAMP3/CD63) was among the highly upregulated genes during HSV-2 infection of human vaginal epithelial cell line VK2, leading us to posit that LAMP3/CD63 may play a role in HSV-2 infection. Consequently, we generated two gene-altered VK2-derived cell lines, a LAMP3-overexpressed (OE) line and a LAMP3 knockout (KO) line. The wild-type VK2 and the LAMP3 OE and KO cell lines were grown in air-liquid interface (ALI) cultures for 7 days and infected with HSV-2. Twenty-four hours postinfection, LAMP3 OE cells produced and released significantly higher numbers of HSV-2 virions than wild-type VK2 cells, while virus production was greatly attenuated in LAMP3 KO cells, indicating a functional association between LAMP3/CD63 expression and HSV-2 replication. Fluorescence microscopy of HSV-2-infected cells revealed that HSV-2 colocalized with LAMP3 in both early endosomes and lysosomal compartments. In addition, blocking endosomal maturation or late endosomal/lysosomal fusion using specific inhibitors resulted in reduced HSV-2 replication in VK2 cells. Similarly, LAMP3 KO cells exhibited very low viral entry and association with endosomes, while LAMP3 OE cells demonstrated large amounts of virus that colocalized with LAMP3/CD63 in endosomes and lysosomes. IMPORTANCE Collectively, these results showed that HSV-2 is taken up by human vaginal epithelial cells through an endosomal-lysosomal pathway in association with LAMP3, which plays a crucial role in the enhancement of HSV-2 replication. These findings provide the basis for the future design of antiviral agents for prophylactic measures against HSV-2 infection.

Rab3a, a small GTP-binding protein, is required for the stabilization of the murine leukaemia virus Gag protein

We recently identified a CD63-interacting protein to understand the role of CD63 in virion production of the human immunodeficiency virus type 1, and we have found that Rab3a forms a complex with CD63. In this study, we analysed the effect of Rab3a on virion production of the murine leukaemia virus (MLV), which is another member of the retrovirus family. We found that Rab3a silencing induced lysosomal degradation of the MLV Gag protein, and recovery of the Rab3a expression restored the level of the Gag protein through a complex formation of MLV Gag and Rab3a, indicating that Rab3a is required for MLV Gag protein expression. In contrast, CD63 silencing decreased the infectivity of released virions but had no effect on virion production, indicating that CD63 facilitates the infectivity of released MLV particles. Although Rab3a induced CD63 degradation in uninfected cells, the complex of MLV Gag and Rab3a suppressed the Rab3a-mediated CD63 degradation in MLV-infected cells. Finally, we found that the MLV Gag protein interacts with Rab3a to stabilize its own protein and CD63 that facilitates the infectivity of released MLV particles. Considering the involvement of Rab3a in lysosome trafficking to the plasma membrane, it may also induce cell surface transport of the MLV Gag protein.

Antivirus activity, but not thiolreductase activity, is conserved in interferon-gamma-inducible GILT protein in arthropod

We have previously reported that gamma-interferon inducible lysosomal thiolreductase (GILT) functions as a host defense factor against retroviruses by digesting disulfide bonds on viral envelope proteins. GILT is widely conserved even in plants and fungi as well as animals. The thiolreductase active site of mammalian GILT is composed of a CXXC amino acid motif, whereas the C-terminal cysteine residue is changed to serine in arthropods including shrimps, crabs, and flies. GILT from Penaeus monodon (PmGILT) also has the CXXS motif instead of the CXXC active site. We demonstrate here that a human GILT mutant (GILT C75S) with the CXXS motif and PmGILT significantly inhibit amphotropic murine leukemia virus vector infection in human cells without alterning its expression level and lysosomal localization, showing that the C-terminal cysteine residue of the active site is not required for the antiviral activity. We have reported that human GILT suppresses HIV-1 particle production by digestion of disulfide bonds on CD63. However, GILT C75S mutant and PmGILT did not digest CD63 disulfide bonds, and had no effect on HIV-1 virion production, suggesting that they do not have thiolreductase activity. Taken together, this study found that antiviral activity, but not thiolreductase activity, is conserved in arthropod GILT proteins. This finding provides a new insight that the common function of GILT is antiviral activity in many animals.

Tetraspanins: Host Factors in Viral Infections

Tetraspanins are transmembrane glycoproteins that have been shown increasing interest as host factors in infectious diseases. In particular, they were implicated in the pathogenesis of both non-enveloped (human papillomavirus (HPV)) and enveloped (human immunodeficiency virus (HIV), Zika, influenza A virus, (IAV), and coronavirus) viruses through multiple stages of infection, from the initial cell membrane attachment to the syncytium formation and viral particle release. However, the mechanisms by which different tetraspanins mediate their effects vary. This review aimed to compare and contrast the role of tetraspanins in the life cycles of HPV, HIV, Zika, IAV, and coronavirus viruses, which cause the most significant health and economic burdens to society. In doing so, a better understanding of the relative contribution of tetraspanins in virus infection will allow for a more targeted approach in the treatment of these diseases.

Molecular cloning and characterization of CD63 in common carp infected with koi herpesvirus

CD63 is a member of the four-transmembrane-domain protein superfamily and is the first characterized tetraspanin protein. In the present study, we cloned the common carp (Cyprinus Carpio) CD63 (ccCD63) sequence and found that the ccCD63 ORF contained 711 bp and encoded a protein of 236 amino acids. Homology analysis revealed that the complete ccCD63 sequence had 84.08% amino acid similarity to CD63 of Sinocyclocheilus anshuiensis. Subcellular localization analysis revealed that ccCD63 was localized in the cytoplasm. Quantitative real-time PCR (qRT-PCR) analysis indicated that ccCD63 was expressed in the gill, intestine, liver, spleen, brain and kidney, with higher expression in spleen and brain tissues than in the other examined tissues. After koi herpesvirus (KHV) infection, these tissues exhibited various expression levels of ccCD63. The expression level was the lowest in the liver and highest in the brain; the expression level in the brain was 8.7-fold higher than that in the liver. Furthermore, knockdown of ccCD63 promoted KHV infection. Moreover, ccCD63 was correlated with the regulation of RIG-I/MAVS/TRAF3/TBK1/IRF3 and may be involved in the antiviral response through the RIG-I viral recognition signalling pathway in a TRAF3/TBK1-dependent manner. Taken together, our results suggested that ccCD63 upregulated the interaction of KHV with the host immune system and suppressed the dissemination of KHV.

Generation of cd63-deficient zebrafish to analyze the role of cd63 in viral infection

The tetraspanin superfamily proteins are transmembrane proteins identified in a diverse range of eukaryotic organisms. Tetraspanins are involved in a variety of essential biological functions, including cell differentiation, adhesion, migration, signal transduction, intracellular trafficking, and immune responses. For an infection to occur, viruses must interact with various cell surface components, including receptors and signaling molecules. Tetraspanin CD63 is involved in the organization of the cell membrane and trafficking of cellular transmembrane proteins that interact with many viruses. In this study, the cd63 gene was characterized by studying its expression and function in a zebrafish model. The functional domains and structural features of Cd63, such as the Cys-Cys-Gly (CCG) motif in the large extracellular loop and cysteine residues, are conserved in zebrafish. We confirmed that cd63 was expressed in immune system organs, such as the axial vein and pronephric duct, during the embryonic development of zebrafish. To better understand the role of cd63 in the zebrafish immune system, we established cd63-deficient zebrafish lines using the clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9 (CRISPR/Cas9) system. A 19 bp insertion mutation was generated in single guide RNA (sgRNA) target sequence of exon 3 of the cd63 gene, to create a pre-mature stop codon. We then analyzed the expression of cd63-related genes cxcr4a and cxcr4b in wild type (WT) and cd63-deficient zebrafish. We believe our study provides an important model that could be used to investigate the roles of cd63 in viral infection in vivo.

Tetraspanins: useful multifunction proteins for the possible design and development of small-molecule therapeutic tools

Tetraspanins constitute a well-conserved superfamily of four-span small membrane proteins (TM4SF), with >30 members in humans, with important roles in numerous mechanisms of cell biology. Moreover, tetraspanins associate with either specific partner proteins or another tetraspanin, generating a network of interactions involved in cell and membrane compartmentalization and having a role in cellular development, proliferation, activation, motility, and membrane fusions. Therefore, tetraspanins are considered regulators of cellular signaling and are often depicted as 'molecular facilitators'. In view of these many physiological functions, it is likely that these molecules are important actors in pathological processes. In this review, we present the main characteristics of this superfamily, providing a more detailed description of some significant representatives and discuss their relevance as potential targets for the design and development of small-molecule therapeutics in different pathologies.

HPV caught in the tetraspanin web?

Tetraspanins are master organizers of the cell membrane. Recent evidence suggests that tetraspanins themselves may become crowded by virus particles and that these crowds/aggregates co-internalize with the viral particles. Using microscopy, we studied human papillomavirus (HPV) type 16-dependent aggregates on the cell surface of tetraspanin overexpressing keratinocytes. We find that aggregates are (1) rich in at least two different tetraspanins, (2) three-dimensional architectures extending up to several micrometers into the cell, and (3) decorated intracellularly by filamentous actin. Moreover, in cells not overexpressing tetraspanins, we note that obscurin-like protein 1 (OBSL1), which is thought to be a cytoskeletal adaptor, associates with filamentous actin. We speculate that HPV contact with the cell membrane could trigger the formation of a large tetraspanin web. This web may couple the virus contact site to the intracellular endocytic actin machinery, possibly involving the cytoskeletal adaptor protein OBSL1. Functionally, such a tetraspanin web could serve as a virus entry platform, which is co-internalized with the virus particle.

Rapid Isolation of Rare Isotype-Switched Hybridoma Variants: Application to the Generation of IgG2a and IgG2b MAb to CD63, a Late Endosome and Exosome Marker

CD63, a member of the tetraspanin superfamily, is used as a marker of late endosomes and lysosome-related organelles, as well as a marker of exosomes. Here, we selected rare isotype variants of TS63 by sorting hybridoma cells on the basis of their high expression of surface immunoglobulins of the IgG2a and IgG2b subclass. Pure populations of cells secreting IgG2a and IgG2b variants of TS63 (referred to as TS63a and TS63b) were obtained using two rounds of cell sorting and one limited dilution cloning step. We validate that these new TS63 variants are suitable for co-labeling with mAb of the IgG1 subclass directed to other molecules, using anti mouse subclass antibodies, and for the labeling of exosomes through direct binding to protein A-coated gold particles. These mAbs will be useful to study the intracellular localization of various proteins and facilitate electron microscopy analysis of CD63 localization.

Anti-human CD63 monoclonal antibody COS3A upregulates monocyte-induced IL-10 excretion leading to diminution of CD3-mediated T cell response

Human CD63 has been reported to play a role either as an inhibitor or as a co-stimulator of T- cell responses, although the mechanism of this is unclear. In this study, an anti-human CD63 monoclonal antibody (mAb) COS3A was used to monitor the role of CD63 in T-cell activation. MAb COS3A could inhibit CD3-mediated T-cell proliferation and CD25 expression in peripheral blood mononuclear cells (PBMCs), used as a study model, but the suppressive effect was not observed when purified T-cells were used instead of PBMCs. The inhibitory phenomenon was associated with downregulation of IL-2 and IFN-γ by T-cells, but upregulation of IL-10 by monocytes. Neutralizing IL-10 with anti-IL-10 mAb improved the T-cell response, indicating the role of IL-10 in T-cell suppression. In this study, monocytes were demonstrated to play a role in impeding T-cell activation by the anti-CD63 mAb COS3A. This is the first evidence that anti-CD63 mAb induces IL-10 secretion by monocytes, which later play a role in T-cell hypo-responsiveness.

Proteomic Responses Under Cold Stress Reveal Unique Cold Tolerance Mechanisms in the Pacific White Shrimp (Litopenaeus vannamei)

The Pacific white shrimp (Litopenaeus vannamei), one of the most widely cultured shrimp species in the world, often suffers from cold stress. To understand the molecular mechanism of cold tolerance in Pacific white shrimp, we conducted a proteomic analysis on two contrasting shrimp cultivars, namely, cold-tolerant Guihai2 (GH2) and cold-sensitive Guihai1 (GH1), under normal temperature (28°C), under cold stress (16°C), and during recovery to 28°C. In total, 3,349 proteins were identified, among which 2,736 proteins were quantified. Based on gene ontology annotations, differentially expressed proteins largely belonged to biological processes, cellular components, and molecular functions. KEGG pathway annotations indicated that the main changes were observed in the lysosome, ribosomes, and oxidative phosphorylation. Subcellular localization analysis showed a significant increase in proteins present in cytosol, extracellular regions, and mitochondria. Combining enrichment-based clustering analysis and qRT-PCR analysis, we found that glutathione S-transferase, zinc proteinase, m7GpppX diphosphatase, AP2 transcription complex, and zinc-finger transcription factors played a major role in the cold stress response in Pacific white shrimp. Moreover, structure proteins, including different types of lectin and DAPPUDRAFT, were indispensable for cold stress tolerance of the Pacific white shrimp. Results indicate the molecular mechanisms of the Pacific white shrimp in response to cold stress and provide new insight into breeding new cultivars with increased cold tolerance.

Tetraspanins, Another Piece in the HIV-1 Replication Puzzle

Despite the great research effort placed during the last decades in HIV-1 study, still some aspects of its replication cycle remain unknown. All this powerful research has succeeded in developing different drugs for AIDS treatment, but none of them can completely remove the virus from infected patients, who require life-long medication. The classical approach was focused on the study of virus particles as the main target, but increasing evidence highlights the importance of host cell proteins in HIV-1 cycle. In this context, tetraspanins have emerged as critical players in different steps of the viral infection cycle. Through their association with other molecules, including membrane receptors, cytoskeletal proteins, and signaling molecules, tetraspanins organize specialized membrane microdomains called tetraspanin-enriched microdomains (TEMs). Within these microdomains, several tetraspanins have been described to regulate HIV-1 entry, assembly, and transfer between cells. Interestingly, the importance of tetraspanins CD81 and CD63 in the early steps of viral replication has been recently pointed out. Indeed, CD81 can control the turnover of the HIV-1 restriction factor SAMHD1. This deoxynucleoside triphosphate triphosphohydrolase counteracts HIV-1 reverse transcription (RT) in resting cells via its dual function as dNTPase, catalyzing deoxynucleotide triphosphates into deoxynucleosides and inorganic triphosphate, and as exonuclease able to degrade single-stranded RNAs. SAMHD1 has also been related with the detection of viral nucleic acids, regulating the innate immune response and would promote viral latency. New evidences demonstrating the ability of CD81 to control SAMHD1 expression, and as a consequence, HIV-1 RT activity, highlight the importance of TEMs for viral replication. Here, we will briefly review how tetraspanins modulate HIV-1 infection, focusing on the latest findings that link TEMs to viral replication.

Tetraspanin Assemblies in Virus Infection

Tetraspanins (Tspans) are a family of four-span transmembrane proteins, known as plasma membrane “master organizers.” They form Tspan-enriched microdomains (TEMs or TERMs) through lateral association with one another and other membrane proteins. If multiple microdomains associate with each other, larger platforms can form. For infection, viruses interact with multiple cell surface components, including receptors, activating proteases, and signaling molecules. It appears that Tspans, such as CD151, CD82, CD81, CD63, CD9, Tspan9, and Tspan7, coordinate these associations by concentrating the interacting partners into Tspan platforms. In addition to mediating viral attachment and entry, these platforms may also be involved in intracellular trafficking of internalized viruses and assist in defining virus assembly and exit sites. In conclusion, Tspans play a role in viral infection at different stages of the virus replication cycle. The present review highlights recently published data on this topic, with a focus on events at the plasma membrane. In light of these findings, we propose a model for how Tspan interactions may organize cofactors for viral infection into distinct molecular platforms.

Multiple Inhibitory Factors Act in the Late Phase of HIV-1 Replication: a Systematic Review of the Literature

The use of lentiviral vectors for therapeutic purposes has shown promising results in clinical trials. The ability to produce a clinical-grade vector at high yields remains a critical issue. One possible obstacle could be cellular factors known to inhibit human immunodeficiency virus (HIV). To date, five HIV restriction factors have been identified, although it is likely that more factors are involved in the complex HIV-cell interaction. Inhibitory factors that have an adverse effect but do not abolish virus production are much less well described. Therefore, a gap exists in the knowledge of inhibitory factors acting late in the HIV life cycle (from transcription to infection of a new cell), which are relevant to the lentiviral vector production process. The objective was to review the HIV literature to identify cellular factors previously implicated as inhibitors of the late stages of lentivirus production. A search for publications was conducted on MEDLINE via the PubMed interface, using the keyword sequence "HIV restriction factor" or "HIV restriction" or "inhibit HIV" or "repress HIV" or "restrict HIV" or "suppress HIV" or "block HIV," with a publication date up to 31 December 2016. Cited papers from the identified records were investigated, and additional database searches were performed. A total of 260 candidate inhibitory factors were identified. These factors have been identified in the literature as having a negative impact on HIV replication. This study identified hundreds of candidate inhibitory factors for which the impact of modulating their expression in lentiviral vector production could be beneficial.

Intravenously delivered mesenchymal stem cell-derived exosomes target M2-type macrophages in the injured spinal cord

In a previous report we showed that intravenous infusion of bone marrow-derived mesenchymal stem cells (MSCs) improved functional recovery after contusive spinal cord injury (SCI) in the non-immunosuppressed rat, although the MSCs themselves were not detected at the spinal cord injury (SCI) site [1]. Rather, the MSCs lodged transiently in the lungs for about two days post-infusion. Preliminary studies and a recent report [2] suggest that the effects of intravenous (IV) infusion of MSCs could be mimicked by IV infusion of exosomes isolated from conditioned media of MSC cultures (MSCexos). In this study, we assessed the possible mechanism of MSCexos action on SCI by investigating the tissue distribution and cellular targeting of DiR fluorescent labeled MSCexos at 3 hours and 24 hours after IV infusion in rats with SCI. The IV delivered MSCexos were detected in contused regions of the spinal cord, but not in the noninjured region of the spinal cord, and were also detected in the spleen, which was notably reduced in weight in the SCI rat, compared to control animals. DiR "hotspots" were specifically associated with CD206-expressing M2 macrophages in the spinal cord and this was confirmed by co-localization with anti-CD63 antibodies labeling a tetraspanin characteristically expressed on exosomes. Our findings that MSCexos specifically target M2-type macrophages at the site of SCI, support the idea that extracellular vesicles, released by MSCs, may mediate at least some of the therapeutic effects of IV MSC administration.

Single-cell analysis identifies cellular markers of the HIV permissive cell

Cellular permissiveness to HIV infection is highly heterogeneous across individuals. Heterogeneity is also found across CD4+ T cells from the same individual, where only a fraction of cells gets infected. To explore the basis of permissiveness, we performed single-cell RNA-seq analysis of non-infected CD4+ T cells from high and low permissive individuals. Transcriptional heterogeneity translated in a continuum of cell states, driven by T-cell receptor-mediated cell activation and was strongly linked to permissiveness. Proteins expressed at the cell surface and displaying the highest correlation with T cell activation were tested as biomarkers of cellular permissiveness to HIV. FACS sorting using antibodies against several biomarkers of permissiveness led to an increase of HIV cellular infection rates. Top candidate biomarkers included CD25, a canonical activation marker. The combination of CD25 high expression with other candidate biomarkers led to the identification of CD298, CD63 and CD317 as the best biomarkers for permissiveness. CD25highCD298highCD63highCD317high cell population showed an enrichment of HIV-infection of up to 28 fold as compared to the unsorted cell population. The purified hyper-permissive cell subpopulation was characterized by a downregulation of interferon-induced genes and several known restriction factors. Single-cell RNA-seq analysis coupled with functional characterization of cell biomarkers provides signatures of the "HIV-permissive cell".

Rab3a-Bound CD63 Is Degraded and Rab3a-Free CD63 Is Incorporated into HIV-1 Particles

CD63, a member of the tetraspanin family, is involved in virion production by human immunodeficiency virus type 1 (HIV-1), but its mechanism is unknown. In this study, we showed that a small GTP-binding protein, Rab3a, interacts with CD63. When Rab3a was exogenously expressed, the amounts of CD63 decreased in cells. The Rab3a-mediated reduction of CD63 was suppressed by lysosomal and proteasomal inhibitors. The amount of CD63 was increased by reducing the endogenous Rab3a level using a specific shRNA. These results indicate that Rab3a binds to CD63 to induce the degradation of CD63. Rab3a is thought to be involved in exocytosis, but we found that another function of Rab3a affects the fate of CD63 in lysosomes. CD63 interacted with Rab3a and was incorporated into HIV-1 particles. However, Rab3a was not detected in HIV-1 virions, thereby indicating that Rab3a-free CD63, but not Rab3a-bound CD63, is incorporated into HIV-1 particles. Overexpression or silencing of Rab3a moderately reduced HIV-1 virion formation. Overexpression of Rab3a decreased CD63 levels, but did not affect the incorporation of CD63 into HIV-1 particles. This study showed that Rab3a binds to CD63 to induce the degradation of CD63, and only Rab3a-free CD63 is incorporated into HIV-1 particles.

The Role of Exosomal VP40 in Ebola Virus Disease

Ebola virus (EBOV) can cause a devastating hemorrhagic disease, leading to death in a short period of time. After infection, the resulting EBOV disease results in high levels of circulating cytokines, endothelial dysfunction, coagulopathy, and bystander lymphocyte apoptosis in humans and nonhuman primates. The VP40 matrix protein of EBOV is essential for viral assembly and budding from the host cell. Recent data have shown that VP40 exists in the extracellular environment, including in exosomes, and exosomal VP40 can impact the viability of recipient immune cells, including myeloid and T cells, through the regulation of the RNAi and endosomal sorting complexes required for transport pathways. In this study, we discuss the latest findings of the impact of exosomal VP40 on immune cells in vitro and its potential implications for pathogenesis in vivo.

Tetraspanin CD151 Promotes Initial Events in Human Cytomegalovirus Infection

Human cytomegalovirus (HCMV), a betaherpesvirus, can cause life-threatening disease in immunocompromised individuals. Viral envelope glycoproteins that mediate binding to and penetration into target cells have been identified previously. In contrast, cellular proteins supporting HCMV during entry are largely unknown. In order to systematically identify host genes affecting initial steps of HCMV infection, a targeted RNA interference screen of 96 cellular genes was performed in endothelial cells by use of a virus strain expressing the full set of known glycoprotein H and L (gH/gL) complexes. The approach yielded five proviral host factors from different protein families and eight antiviral host factors, mostly growth factor receptors. The tetraspanin CD151 was uncovered as a novel proviral host factor and was analyzed further. Like endothelial cells, fibroblasts were also less susceptible to HCMV infection after CD151 depletion. Virus strains with different sets of gH/gL complexes conferring either broad or narrow cell tropism were equally impaired. Infection of CD151-depleted cells by a fluorescent virus with differentially labeled capsid and envelope proteins revealed a role of CD151 in viral penetration but not in adsorption to the cell. In conclusion, the tetraspanin CD151 has emerged as a novel host factor in HCMV entry and as a putative antiviral target.

IMPORTANCE

At present, the events at the virus-cell interface and the cellular proteins involved during the HCMV entry steps are scarcely understood. In this study, several host factors with putative roles in this process were identified. The tetraspanin CD151 was discovered as a previously unrecognized proviral host factor for HCMV and was found to support viral penetration into the target cells. The findings of this study shed light on the cellular contribution during the initial steps of HCMV infection and open a new direction in HCMV research.

Profiling of Host Cell Response to Successive Canine Parvovirus Infection Based on Kinetic Proteomic Change Identification

Canine parvovirus (CPV) reproduces by co-opting the resources of host cells, inevitably causing cytotoxic effects to the host cells. Feline kidney F81 cells are sensitive to CPV infection and show disparate growing statuses at different time points post-infection. This study analysed the response of F81 cells to CPV infection at successive infection time points by iTRAQ-based quantitative proteomics. Differentially expressed proteins (DEPs) during 60 h of infection and at selected time points post-infection were identified by an analysis of variance test and a two-tailed unpaired t test, respectively. DEPs with similar quantitative changes were clustered by hierarchical clustering and analysed by gene ontology enrichment, revealing that 12 h and 60 h post-infection were the optimal times to analyse the autonomous parvovirus replication and apoptosis processes, respectively. Using the Metacore^TM database, 29 DEPs were enriched in a network involved in p53 regulation. Besides, a significantly enriched pathway suggests that the CPV-induced cytopathic effect was probably due to the deficiency of functional CFTR caused by CPV infection. This study uncovered the systemic changes in key cellular factors involved in CPV infection and help to understand the molecular mechanisms of the anti-cancer activity of CPV and the cytopathic effects induced by CPV infection.

Rab27a controls HIV-1 assembly by regulating plasma membrane levels of phosphatidylinositol 4,5-bisphosphate

During the late stages of the HIV-1 replication cycle, the viral polyprotein Pr55(Gag) is recruited to the plasma membrane (PM), where it binds phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) and directs HIV-1 assembly. We show that Rab27a controls the trafficking of late endosomes carrying phosphatidylinositol 4-kinase type 2 α (PI4KIIα) toward the PM of CD4(+) T cells. Hence, Rab27a promotes high levels of PM phosphatidylinositol 4-phosphate and the localized production of PI(4,5)P2, therefore controlling Pr55(Gag) membrane association. Rab27a also controls PI(4,5)P2 levels at the virus-containing compartments of macrophages. By screening Rab27a effectors, we identified that Slp2a, Slp3, and Slac2b are required for the association of Pr55(Gag) with the PM and that Slp2a cooperates with Rab27a in the recruitment of PI4KIIα to the PM. We conclude that by directing the trafficking of PI4KIIα-positive endosomes toward the PM, Rab27a controls PI(4,5)P2 production and, consequently, HIV-1 replication.

Vpu is the main determinant for tetraspanin downregulation in HIV-1-infected cells

Tetraspanins constitute a family of cellular proteins that organize various membrane-based processes. Several members of this family, including CD81, are actively recruited by HIV-1 Gag to viral assembly and release sites. Despite their enrichment at viral exit sites, the overall levels of tetraspanins are decreased in HIV-1-infected cells. Here, we identify Vpu as the main viral determinant for tetraspanin downregulation. We also show that reduction of CD81 levels by Vpu is not a by-product of CD4 or BST-2/tetherin elimination from the surfaces of infected cells and likely occurs through an interaction between Vpu and CD81. Finally, we document that Vpu-mediated downregulation of CD81 from the surfaces of infected T cells can contribute to preserving the infectiousness of viral particles, thus revealing a novel Vpu function that promotes virus propagation by modulating the host cell environment.

IMPORTANCE

The HIV-1 accessory protein Vpu has previously been shown to downregulate various host cell factors, thus helping the virus to overcome restriction barriers, evade immune attack, and maintain the infectivity of viral particles. Our study identifies tetraspanins as an additional group of host factors whose expression at the surfaces of infected cells is lowered by Vpu. While the downregulation of these integral membrane proteins, including CD81 and CD82, likely affects more than one function of HIV-1-infected cells, we document that Vpu-mediated lowering of CD81 levels in viral particles can be critical to maintaining their infectiousness.